Fine Tuning of the Electronic Coupling between Metal Centers in Cyano-Bridged Mixed-Valent Trinuclear Complexes

Abstract

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.

Structural and Spectroscopic Evidence of Strong Electronic Delocalization through a Cyanido Bridge in a Mixed‐Valence Os–Ru Complex

“…The spectrum of 1 4-in acetonitrile closely resembles that observed for the Fe III analogue, [10] with a metal-to-ligand charge transfer (MLCT) transition, Ru II Ǟπ*(dmap), at 29100 cm -1 (ε = 30200 m -1 cm -1 ) and an MMЈCT transition at 8500 cm -1 (ε = 14200 m -1 cm -1 ) (Figure 2). The main difference relies on the intensity of the MMЈCT band, which suggests a stronger interaction between the metal ions through the cyanido bridge as expected for the replacement of a first-row transition metal ion for a third-row one.…”

“…The crystal structure of 1 4- (Figure 1) displays a linear and totally eclipsed configuration of the three coordination spheres, where the three metal centres are connected by two cyanido bridges as observed in other related trinuclear complexes previously reported. [10] Remarkably, the Ru-N(dmap) mean bond length (2.089 Å) is between the value observed for this Ru-N bond length in the Ru II cyanide trinuclear complex trans-…”

“…[10] ) and the value observed for the Ru-N bond length in the Ru III -containing trinuclear compound [(dmap) 4 Figure S1). Additionally, the C-N bridge bond is slightly longer [1.170(9) Å in 1 4- vs. 2.043 Å for the mean cis Os-C bond length].…”

Electronic Conduction in Photoactive Metallo‐wires

“…Knowledge of V DA is of great importance for the rational design of new bridges and for understanding the role of the bridge in electron-exchange processes [107][108][109][110]. According to super-exchange theory, the coupling element will decrease exponentially with increasing distance (R) between donor and acceptor [111].…”

“…The coupling element can also be related to the energy gap ðE DB Þ between orbitals on the donor and on the bridge (Fig. 2.12) [107][108][109][110].…”